Land granted that is held between two forces (4)
I believe the answer is:
fief
'land granted' is the definition.
(as in a fiefdom)
'that is held between two forces' is the wordplay.
'two forces' can mean two substitutions for 'force'.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'held between' means one lot of letters goes inside another.
'force' becomes 'F' (symbol used in Physics e.g. f=ma).
'force' becomes 'F' (symbol used in Physics e.g. f=ma).
'f'+'f'='ff'
'ie' inserted inside 'ff' is 'FIEF'.
(Other definitions for fief that I've seen before include "Feudal holding" , "Grant of land" , "Land held in return for feudal or military service" , "Sphere of control" .)